Cracking
A Meteorite Mystery

by
Robert Sanders

A new and detailed picture of how the Sun and planets formed from a swirling
mass of gas and dust also explains the puzzling composition of the most
common meteorites, the chondrites, which date from the formation of the
solar system some 4.5 billion years ago.

Ever since the first meteorite was cracked open over a hundred years
ago astronomers and meteoriticists have puzzled over the paradoxical mix
of minerals-some heated to high temperatures and others formed at cold temperatures.

Based on a theoretical model of how clouds of gas and dust condense into
stars and planets, astronomer Frank Shu and colleagues Typhoon Lee and Hsien
Shang propose that stars like the Sun recycle some of the dust falling into
the star, throwing it out from the center in a fiery spray that seeds the
colder matter with small "chondrules" or beads of melted rock.

Chondrules eventually coalesced with the remaining cold matter in the
planetary disk to form asteroids, which are thought to have aggregated into
the planets.

The theory predicts that chondrules of only a certain size, ranging from
a millimeter to about a centimeter, would fall back into the disk, in agreement
with the size of chondrules found in the most common types of meteorites.

The theory has broader implications, since the formation of large asteroids
and planets may not have been possible without these droplets of melted
star dust in the early planetary nebula. Plus it explains why the Earth
and many asteroids are deficient in certain elements.

"This is the first theory to explain all the disparate features
of chondrites, such as the narrow range of sizes of chondrules and why their
composition is different from the Sun," Shu says. "And because
planets formed from chondritic asteroids, it also explains the composition
of the planets."